Why is it important to ensure flow separation does not occur in the back of a vehicle or wing? I know that when flow separation occurs its when you have a large differential pressure which in turn cause turbulence and drag. I understand that with more drag you have less efficiency. But if it it occurs behind the object (tail end of the car for example) wouldn't be okay since the object in motion is already in front of the flow separation? How is the wake behind the object physically affects the car or wing?